If I get 3bet and a coldcall, I can pretty much be sure to be behind against at least one of them. So is it okay to call for set value and give up if didn't hit (even if I have an overpair on that flop) ?

Since everybody has at least 100bb, overcalling with JJ is fine preflop.

On that flop i'd usually check/call a bet from BU unless he's really tight, like his range preflop is QQ+/AK and he doesn't c-bet much, in which case we'd be up against a better overpair often and it wouldn't be worth the trouble.

Once SB comes in with a 2.7x 3bet tho, i think we're flipping/dead against him and we also might be behind BU who's still left to act, dumping JJ at that point makes all the sense in the world.